Investigators have no found evidence that the internal fire that sent smoke billowing through a parked and empty Boeing Co. 787 Dreamliner in London on Friday was linked to problematic aircraft batteries, Britain's Air Accidents Investigation Branch said on Saturday.

In a statement, the agency said that "it is clear that this heat damage is remote from the area in which the aircraft main and APU (Auxiliary Power Unit) batteries are located, and, at this stage, there is no evidence of a direct causal relationship."

Ethiopian Airlines was the first to resume flying the 787 after Boeing ordered modifications to the battery systems of all 50 of the aircraft.

But on Friday, one of Ethiopian's four 787s caught fire at London's Heathrow Airport, raising alarms that maybe the battery problems had not been resolved. Boeing had never found the root cause of the overheating. The company's shares fell 5% in the wake of Friday's incident.

British investigators said "physical evidence shows that this event resulted in smoke throughout the fuselage and extensive heat damage in the upper portion of the rear fuselage." The aircraft, dubbed the Queen of Sheba, had been parked for more than eight hours and was a few hours away from taking off again.

Analysts said the fire could have been caused by some other electronic malfunction, including a heater being left on or not enough air conditioning being turned on to cool the plane during maintenance.

Boeing has delivered 68 Dreamliners, as of July 5. More than 900 have been ordered. It didn't appear that any airlines had grounded their 787s.

A Florida-bound Thomson Airways flight returned to England mid-voyage Friday because of an unspecified "technical issue." But the airline said on Saturday that it fixed the minor problem and would resume flights.

The U.S. National Transportation Safety Board said Senior Air Safety Investigator Lorenda Ward as well as other NTSB and FAA investigators would join in helping British authorities determine the cause of the Ethiopian Airlines fire.